1. Field of the Invention
The present invention relates generally to printed circuit board technology, and more particularly to the provision and use of new tin or tin-lead stripping solutions to achieve an improved stripping operation.
2. Description of the Prior Art
Chemical treatments utilizing aqueous solutions consisting of a fluoride source, such as hydrofluoric acid, fluoboric acid, ammonium bifluoride, or mixtures thereof, and hydrogen peroxide as an oxidizing agent are commonly used by the printed circuit industry for stripping tin or tin-lead plating resist from printed circuit board taps or the like. However, these aqueous solutions are sufficiently aggressive that after complete stripping of the tin, tin-lead, a significant, undesirable attack on the underlying copper substrate occurs along with excessive discoloration of the copper. The attack on the underlying copper substrate reduces the copper thickness and correspondingly, the current-carrying capacity of the board. Secondly, the excessive discoloration of the copper requires additional cleaning pretreatment prior to plating of the circuit board tabs with nickel, tin-nickel, gold, or rhodium. Without this additional surface cleaning, poor adhesion of the subsequent plated metals can occur.
There are many methods known to improve the copper substrate appearance, however, these methods require steps which are an addition to, or at least, a continuation of the basic tin/tin-lead stripping process. Improvements in the performance of the basic fluoride-peroxide tin/tin-lead stripping process which result in a reduction in the usual discoloration and attack on underlying copper substrates have thus long been desired.